Add more about undefined/junk cases
Add more about undefined/junk cases

This commit adds some of information about undefined/junk
cases that Norm raised in:
https://github.com/metamath/set.mm/pull/1406 ;

Over the years we have repeatedly had questions/discussions
about undefined/junk cases. This tries to document *why* things
are the way they are, to answer those questions ahead of time.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm